Use SSize_t when extending the stack

Use SSize_t when extending the stack

(I am referring to what is usually known simply as The Stack.)

This partially fixes #119161.

By casting the argument to int, we can end up truncating/wrapping
it on 64-bit systems, so EXTEND(SP, 2147483648) translates into
EXTEND(SP, -1), which does not extend the stack at all.  Then writing
to the stack in code like ()=1..1000000000000 goes past the end of
allocated memory and crashes.

I can’t really write a test for this, since instead of crashing it
will use more memory than I have available (and then I’ll start for-
getting things).